Global warming, petroleum depletion and energy security have been the main driving forces for the development of renewable fuels that can replace the petroleum-derived fuels, such as gasoline and diesel. Ethanol is currently the most commonly used renewable automobile fuel. It is largely produced by fermentation of sugar or starch-containing feedstocks, such as cane sugar, corn and wheat. However, the supply of these crops is relatively limited, and many of them can be considered as a human food resource. Another disadvantage is that the production of ethanol from most of these raw materials gives a relatively low net energy gain and a low renewable CO2-efficiency, i.e. the amount of fossil CO2 produced throughout the production chain when producing ethanol from these materials is high. Lignocellulose is a more abundant and less expensive raw material with the potential to give a higher net energy gain.
Lignocellulose is primarily composed of cellulose, hemicellulose and lignin. Cellulose is composed of polysaccharide chains of several hundred to over ten thousand linked glucose units, whereas hemicellulose is a branched polysaccharide composed of xylose, other pentose sugars and various hexose sugars. Cellulose and hemicellulose are tightly associated to lignin, a polyphenolic compound that ties the cellulose and hemicellulose polymers together, thus providing the lignocellulosic material with rigidity and mechanical strength.
In the production of ethanol from lignocellulosic materials, various pretreatment and hydrolysis steps are used to degrade the cellulose and hemicellulose polysaccharides in the lignocellulose to monosaccharides. Microorganisms can then be used to ferment the monosaccharides to ethanol. Pretreatment of lignocellulosic feedstocks can be carried out physically (mechanical comminution, pyrolysis), chemically (dilute acid, alkaline pretreatment), physicochemically (steam explosion), and biologically (fungal delignification). These methods open up the lignocellulosic multicomponent matrix and render the carbohydrate components more accessible to hydrolytic enzymes. Besides effective cellulose liberation, an ideal pretreatment has to minimize the formation of degradation products because of their inhibitory effects on subsequent hydrolysis and fermentation processes. However, several approaches for reducing the negative impact of inhibitors have been suggested.
The need for high energy, chemicals and corrosion-resistant, high-pressure reactors make pretreatment to one of the most expensive steps in cellulosic ethanol production.
A common pretreatment step is to use an inorganic acid. The spent liquid from such pretreatment needs to be purified. For example, when using SO2 gas, which is considered an inorganic acid in the context of the present disclosure, during the pretreatment, the resulting spent liquid comprises sulphur which needs to be removed and taken care of. Furthermore, using an inorganic acid in the pretreatment step may entail costs and be harmful to the environment.
To summarize, there is a need in the art for improving the pretreatment in the process for production of ethanol.